Rocks, soil and how fossils form
Chemistry · Key Stage 2 · The School
Rocks are grouped by their history, not their colour. IGNEOUS rock cooled from molten rock, so it is often hard and full of crystals — granite and basalt. SEDIMENTARY rock formed as layers of sand, mud or shell were pressed together over a very long time — sandstone, chalk and limestone. METAMORPHIC rock was one of the other two and then got squashed and heated until it changed — slate was mud, marble was limestone. Two rocks can look alike and belong to different groups, which is why you test rather than guess.
Scratch it with a coin: soft rocks mark, hard ones do not. Drip water on it and wait: sandstone drinks it, slate does not, and that is permeability. Look at the grains with a lens — big grains, small grains, or none you can see at all. Drip vinegar on it: limestone and chalk fizz, because the acid reacts with them. Four tests, four pieces of evidence, and none of them is an opinion about the colour.
The order matters. An animal dies and is buried quickly, usually in mud or sand at the bottom of water. The soft parts rot away. The hard parts — bone, shell, teeth — stay behind. Water carrying dissolved minerals seeps through, and those minerals slowly replace the original material, copying its shape. The layers above press down and turn to rock. Millions of years later the land wears away and the fossil reaches the surface, where somebody finds it.
Follow two animals. A shellfish dies on a muddy seabed and is covered within days: soft parts gone, shell buried, minerals in, fossil out. A bird dies in a wood. It is eaten, scattered, and rained on. Nothing buries it quickly, so nothing is left to fossilise. Quick burial is the step almost everything fails. That is why museum cases are full of shells and bones and nearly empty of worms and jellyfish — not because those animals were rare, but because they rot before the mud arrives. The fossil record is a biased sample, and knowing which way it is biased is part of reading it.